Abstracts

Rationale: Dysregulated hippocampal neurogenesis is a prominent feature of temporal lobe epilepsy (TLE), but the relationship between neurogenesis and epileptogenesis is unclear. Anatomical data indicate that most dentate granule cells (DGCs) generated during or after epileptic insults develop abnormally, including ectopic location, persistent hilar basal dendrites (HBDs) and mossy fiber sprouting, and promote increased excitability. Others appear to integrate normally and may reduce excitability. Studies that eliminate post-insult neurogenesis also show mixed results. Few studies have investigated both anatomical and physiological properties of different populations of DGCs in the same animal model. We recently used a retroviral (RV) GFP reporter to birthdate DGCs with respect to the timing of pilocarpine-induced status epilepticus (SE) in adult rats. We found that DGCs that were mature at SE were anatomically normal, while those born after SE displayed characteristic pathology of DGCs in TLE. To test whether post-SE generated DGCs promote pathological function while established DGCs retain normal function, and how morphological abnormalities influence their behavior, we examined electrophysiological characteristics of age- and morphologically-defined DGC cohorts. Methods: Adult male Sprague-Dawley rats experienced pilocarpine-induced SE at 8 weeks of age. We birth-dated DGCs by bilateral stereotaxic injection of RV-GFP into the dentate gyrus on postnatal day 7 (P7) to label cells that would be mature at SE or on P60 to label cells that were generated after SE. Recordings were made 8-12 weeks after SE from acute hippocampal slices using whole-cell patch-clamp under IR-DIC optics. For TLE tissues, cell morphology was visualized by biocytin fills and assigned post-hoc to one of three categories: granule cell layer (GCL), GCL with HBD, and ectopic. Results: Preliminary results indicated that ectopically integrated cells showed greater repetitive action potential firing than those with HBDs or that appeared normally integrated in TLE. Interestingly, we found that even normally integrated cells in TLE were more excitable than DGCs in control tissue. Intrinsic properties that might influence excitability such as input resistance, membrane potential and action potential (AP) threshold were not different among groups. Conclusions: These findings suggest that most adult-born, abnormally integrated DGCs generated after pilocarpine-induced SE are hyperexcitable and may contribute to recurrent seizures. Even normally integrated adult-born DGCs show abnormal excitability. Experiments are ongoing to compare these findings to DGCs that are mature at the time of SE, and to look specifically at synaptically-driven changes in network function. Supported by an Epilepsy Foundation Predoctoral Fellowship (ALA) and NINDS NS058585 (JMP)